I run a 6600GT, and hands-down it's the best video card I've ever had. Forget balanced. I run on High Quality, then I go further by adding additional specular lights, etc. So technically my quality level is "custom". And the funny thing is that I don't even bother with over-clocking it.
I will notice video lag wherever there are a lot of people in one place, whether that's in a city, tradeskill instance, or out in Antonica. Turn a corner where there are a TON of models to render, and it slows down. But then, unless you're running the most high-end system with all the settings turned down, there's no way around this yet.
Now, that being said, I've never been a "twitch" player. I have a MUCH higher tolerance for lower framerates than many other people. I'm the type of guy that would rather see the nice shiny armor and have a split second of video lag, than have the game look like crap but run the smoothest I've ever seen it. Suffice it to say that if you're looking for a perfectly smooth experience, 100% of the time, particularly during large fights, then you're not going to get it at the high settings I run the game on, on the 6600GT. Now, I WOULD get that type of performance if I ran as low as Balanced or even lower. I'm just not willing to sacrifice *that much* video quality for a slight boost in performance.
Overall: I'm absolutely THRILLED with my 6600GT. Not only does it run EQ2 fantastically, but it eats Doom3, HL2, Riddick, etc, for BREAKFAST!!!
Incidentally, I have the GygaByte version. They've got a great cooling solution on the card, so that might make a difference over other vendors as well.